The present invention relates to a method and a tool for machining the surfaces of workpieces, especially the bore surfaces of internal combustion engines.
The goal of any fine machining operation is, among other things, to produce fully functional surfaces. A typical application for a honing operation is the machining of piston or cylinder bore surfaces in spark-ignition and Diesel engines. Honing involves a cutting or machining with a multi-cutting tool of bound grains or crystals, accompanied by constant surface contact between tool and workpiece. The cutting movement is formed by a rotational movement and a back and forth as reciprocating movement. The honed surface exhibits fine, criss-crossing channels or grooves. In this connection, as with all cutting processes, the material structure is deformed in the region of the edge zones. Via an appropriate enlargement or magnification, one can recognize a surface structure where the cutting edges of the honing grooves are spread and overlap. Partially due to overlapping the cross-sectional areas of the honing grooves directly at the surface are less than below. This surface deformation of the honed bore surface is known in technical circles as "metal nap", and has a disadvantageous impact upon the breaking-in and running conditions of the friction partners piston rings/cylinder bore surface, increases the tendency to form burned channels and to score, and leads to an increased initial oil consumption.
Users of the honing process, especially manufacturers of internal combustion engines, have therefore tried for a long time to produce a surface that is as free of metal nap as possible; however, up to now this has not been possible to any satisfactory extent.
For example, a method of the aforementioned general type has become known where the bore surfaces are electrochemically honed and are subsequently subjected to high pressure spray with a liquid stream or fluid jetting with oil, water and the like.
In this method, not only residues from the electrochemical machining are to be removed but also loosened structure is to be removed; on the other hand, the graphite pockets or clusters that are embedded or incorporated in the structure are not to be destroyed or loosened.
A drawback of this heretofore known method is that the electrochemical machining requires a very high technical outlay and capital expenditure, and places high requirements on the corrosion resistance of the honing unit, thereby considerably increasing the expense of the overall machining process.
It is furthermore known to mechanically hone the bore surfaces and to subsequently undertake a further machining or processing in a spray or fluid jetting unit with a liquid stream under high pressure. Here too considerable capital expenditure is necessary due to the separate units. In addition, the results achieved are not satisfactory in all situations. This is based essentially on the fact that the spray or fluid pressure that is required in order to satisfactorily eliminate the metal nap can directly lead to a destruction of the graphite pockets in the structure, as a result of which it is not possible to achieve an optimum production run of bore surfaces.
It is therefore an object of the present invention to provide a method and tool of the aforementioned general type that avoid the indicated drawbacks and which assure that a surface that is free of metal nap can be obtained without, however, destroying or loosening the graphite pockets that are incorporated in the structure.